1. Field of the Invention
The invention relates to a multi-stack optical data storage medium for recording using a focused radiation beam having a wavelength λ and entering through an entrance face of the medium during recording, comprising:
a first substrate having, on a side thereof:
a first L0 guide groove formed therein, and
a first recording stack L0 comprising a recordable type L0 recording layer, the L0 recording layer having a thickness dL0G in the groove and a thickness dL0L adjacent the groove, and a first reflective layer present between the L0 recording layer and the first substrate;
a second substrate having, on a side thereof:
a second L1 guide groove formed therein, and
a second recording stack L1 comprising a recordable type L1 recording layer, the L1 recording layer having a thickness dL1G in the groove and a thickness dL1L adjacent the groove, said second recording stack being present at a position closer to the entrance face than the L0 recording stack; and
a transparent spacer layer sandwiched between the recording stacks, said transparent spacer layer having a thickness substantially larger than the depth of focus of the focused radiation beam.
The invention also relates to the use of such a medium.
2. Description of the Related Art
An embodiment of an optical recording medium as described in the opening paragraph is known from European Patent Application EP1067535A2, corresponding to U.S. Patent Application Publication No. 2005/0063295. The most common embodiment of the medium is a circular disk.
Regarding the market for optical recording, it is clear that the most important and successful format so far is a write-once format, Compact Disk Recordable (CD-R). Although the take-over in importance by Compact Disk ReWritable (CD-RW) has been predicted since a long time, the actual market size of CD-R media is still at least an order of magnitude larger than for CD-RW. Furthermore, the most important parameter for drives is the maximum write speed for R-media, not for RW. Of course, a possible shift of the market to CD-RW is still possible, e.g., because of Mount Rainier standardization for CD-RW. However, the R-format has been proven very attractive due to its 100% compatibility with read-only compact disk (CD).
Recently, the Digital Versatile Disk (DVD) has gained marketshare as a medium with a much higher data storage capacity than the CD. Presently, this format is available in a read only (ROM) and a rewritable (RW) version. Next to the DVD ReWritable (DVD+RW) standard, a new recordable (R), i.e., write once, DVD+R standard was developed. The new DVD+R standard gets increasing attention as an important support for DVD+RW. A possible scenario is that the end customers have become so familiar with an optical write-once format that they might accept it more easily than a re-writable format.
An issue for both the R and RW formats is the limited capacity, and therefore recording time, because only single-stacked media are present. Note that for DVD-Video, which is a ROM disk, dual-layer media already have a considerable market share. A dual-layer, i.e., dual-stack, DVD+RW disk is probably feasible. However, it has become clear that a fully compatible disk, i.e., within the reflection and modulation specification of the dual-layer DVD-ROM, is very difficult to achieve and requires at least a major breakthrough for the properties of the amorphous/crystalline phase-change materials, which are used as recording layers in, e.g., DVD+RW media. Without a full compatibility, the success of a dual-layer DVD+RW in the market is questionable.
In order to obtain a dual-layer DVD+R medium which is compatible with the dual-layer DVD-ROM standard, the effective reflectivity of both the upper L1 layer and the lower L0 layer should be at least 18%. The term “effective” means that the reflection is measured as the portion of effective light coming back from the medium when both stacks L0 and L1 are present and focusing on L0 and L1, respectively. This implies that the L0 stack as such requires a far higher reflection level of, e.g., more than 50%, preferably more than 60%, because the L1 stack absorbs a substantial portion of the incoming and outgoing light. It should be noted that in this document, the normally used convention of notation of L0 and L1, in which notation L0 is the “closest” stack, i.e., closest to the radiation beam entrance face, has been changed: L0 now is the deepest stack and L1 . . . Ln are stacks closer to the radiation beam entrance face. In EP1067535A2, the following definitions are used: dG1 (corresponding to dL1G in this document) is the thickness of the dye layer in a groove of the first or top information recording/reproduction unit which corresponds to L1, dG2 (corresponding to dL0G in this document) is the thickness of the dye layer in a groove of the second information recording/reproduction unit which corresponds to L0. dL1 (corresponding to dL1L in this document) is the thickness of the dye layer on a land corresponding to L1, dL2 (corresponding to dL0L in this document) is the thickness of the dye layer on a land corresponding to L0. The depth of the grooves corresponding to L1 is d1 but the depth of the grooves d2 corresponding to L0 is defined differently. d2 is the height difference of lands and grooves measured on the dye surface after a dye layer has been coated. dG2, d2 and dL2 are set to approximately 174, 140 and 120 nm. A calculation shows that this corresponds to a groove depth g in the substrate corresponding to L0 of about 194 nm. Measurements by applicants have shown that the inverted L0 stack of the known medium with grooves with a depth of 194 nm has a reflectivity of only 15%-50% of the reflectivity of blank areas (i.e., without grooves). This means that it is impossible to reach the desired 60% reflection level because in order to obtain a dual-layer DVD+R medium which is compatible with the dual-layer DVD-ROM standard, the reflectivity of a light beam focused onto the data track of the lower L0 layer should be sufficiently high (typically >60%, depending on the transmission of the upper L1 layer). From a dual-stack DVD+R production point of view, an inverted L0 layer structure is preferred which means that the recording layer of the L0 stack is present at a side of the reflective layer other than the side of the substrate with groove structure. In EP1067535A2, it is proposed to that dG1 is substantially equal to dG2 and that dL1 is substantially equal to dL2. Furthermore dG1 should be larger than dL1, whereas dG2 should be larger than dL2.